In the current integrated circuit technology, metal films, such as aluminum films and copper films are used as conductors. In conjunction with the metal films, diffusion barrier films are used to avoid diffusion of the metal atoms into other areas of the integrated circuit, such as insulators or active areas, which would have a detrimental effect on the functioning of the circuit. A major problem with the prior art relating to the deposition of metal films on diffusion barrier films or the deposition of diffusion barrier films on metal films is a poor adhesion between the two types of films. In particular, when the second film is deposited by chemical vapor deposition (CVD), the adhesion is problematic. In some cases a deposited metal film may even form a discontinuous coating on a diffusion barrier due to the rapid surface diffusion of the metal. This is particularly problematic for copper films. Although copper is solid at the processing temperatures, the very high diffusion rate of copper atoms results in the formation of copper islands on the diffusion barrier. It seems that copper has high interfacial tension and it is difficult to keep copper in continuous film form.
There have been different types of solutions to the adhesion problem. Srinivas Gandikota et. al. teach in a U.S. Pat. No. 6,362,099 that the barrier surface must be completely free from the presence of oxygen atoms because, according to Gandikota et. al., an oxidized barrier surface causes poor adhesion of copper to the barrier. Even when the interface is kept completely free of oxygen, the adhesion of a copper film onto a diffusion barrier film is not always adequate.
Thick layers of barrier metal oxides, such as TiO2 and Ta2O5, may also cause problems because uncontrolled oxidation of diffusion barriers forms metal oxides that have low density.
It is known that thick copper oxide layers cause adhesion problems because copper oxides are rather soft and they delaminate easily under mechanical stress. Failures in techniques known in the art are possibly caused by the uncontrolled growth of a relatively thick copper oxide layer at the diffusion barrier/copper interface.
In U.S. patent application publication No. 2002/0004293A1 of applicant it is proposed to deposit a copper oxide film onto a barrier film and then reduce the copper oxide film to copper by exposure to an alcohol vapor. This method appeared to be effective and the alcohol vapor reduced the copper oxide film effectively throughout the whole film down to the interface with the diffusion barrier with little or no oxygen left in the copper film. However, this method requires an extra copper reduction step. Further, this method does not provide a solution for the opposite sequence, when a diffusion barrier film needs to be deposited onto a metal film.
It is the objective of the present invention to avoid the disadvantages above and to provide a method and process to achieve a good adhesion between metal films and diffusion barrier films.